Degenerative and traumatic damage to the articular cartilage of the knee joint can result in pain and restricted motion. Knee replacement surgery is frequently utilized to alleviate the pain and restore joint function. An incision is made into the knee joint to expose the bones comprising the joint. Cutting guides are used to guide the removal of the articular surfaces that are to be replaced. Artificial joint components are positioned to replace the resected bone ends in order to establish the desired alignment and mechanics of the joint. In a total knee replacement, all of the articulating compartments of the joint are repaired with prosthetic components. However, often only one compartment of the knee joint, typically the medial compartment, is impaired. Thus, in a unicondylar knee replacement, only the damaged compartment is repaired with prosthetic bearing components.
FIGS. 1-3 illustrate several aspects of implant orientation. FIG. 1 illustrates various axes of the lower limb in the frontal plane. Axes can be defined for each segment of the lower limb. For example, the femur 1 has an anatomic axis 2 coinciding generally with its intramedullary canal. It also has a mechanical axis 4, or load axis, running from the center of the femoral head to the center of the knee. The angle 6 between these two axes 2, 4 in the frontal plane varies within the patient population but is on the order of 4-9°. The two axes 2, 4 are approximately superimposed in the sagittal plane (FIG. 2). Likewise, the tibia 3 has a mechanical axis 5 coinciding generally with its intramedullary canal. The mechanical axis 5 of the tibia runs from the center of the knee to the center of the ankle. The transverse axis, or joint line 8, about which the knee flexes, is parallel to a line through the medial and lateral femoral condyles and parallel to the tibial plateau. Typically, the distal femur and proximal tibia are resected to be parallel to the joint line 8, and thus perpendicular to the mechanical axes 4, 5 as indicated at 10 and 12. The intersection of the femoral and tibial mechanical axes 4, 5 may subtend a small angle relative to one another. However, the angle is small such that the axes 4,5 are approximately collinear and may be treated as collinear for most purposes.
FIG. 2 illustrates the knee joint from the side or sagittal view and various bone cuts that may be made to align implant components. The distal femoral cut 10 is typically made perpendicular to the femoral axes 2, 4 in the sagittal plane. The proximal tibial resection 12 is typically cut to match the natural posterior slope, or rotation, 16 of the proximal tibia relative to the mechanical axes 4, 5. The amount of posterior slope 16 relative to a reference line 18 perpendicular to the mechanical axes 4, 5 varies in the patient population but is on the order of 7°. The distance between the distal femoral cut 10 and proximal tibial cut 12 along the mechanical axes 4, 5 is the extension gap. Other cuts may be made depending on the components that are to be implanted. These include an anterior femoral cut 20, anterior femoral chamfer cut 22, posterior femoral chamfer cut 24, and posterior femoral cut 26. The patella 7 may also be cut 28 to allow for replacement of the patellar articular surface. In a unicondylar knee replacement, only the medial or lateral side of the knee joint is resurfaced. Furthermore, the trochlear, or patellar bearing, surface of the femur is typically left intact in a unicondylar procedure. Unicondylar implant designs vary, but typically only the distal femoral cut 10, posterior femoral chamfer cut 24, and posterior femoral cut 26 are needed to accommodate the unicondylar femoral implant.
FIG. 3 depicts six aspects of component positioning relative to a coordinate system in which the x-axis 30 corresponds approximately to the joint line 8, the z-axis 34 corresponds approximately to the mechanical axes 4 and 5, and the y-axis 32 is normal to the other two. Position along each of these axes is depicted by arrows. Position along the x, y, and z axes determines the medial/lateral (dx) 36, anterior/posterior (dy) 38, and proximal/distal (dz) 40 positioning of components respectively. Rotation about each of these axes is also depicted by arrows. Rotation about the z-axis (rz) 42 corresponds anatomically to external rotation of the femoral component, rotation about the x-axis (rx) 44 corresponds to extension plane rotation, and rotation about the y-axis (ry) 46 corresponds to varus/valgus rotation.